How sealed is the cab enclosure? Holes, rust,etc? Feel the hoses and see if any are hot or at least warm. If there is a great amount of difference ,like it was mentioned in a post above, make sure a shut off valve(s) aren't closed. If they are confirmed to be in the opened position, the heater core is more than likely clogged. Does the heater have a working fan?
Another thing is the engine reaching the set operating temperature (stuck open thermostat will cause no heat)?
As a last resort I had a mr buddy 4K-9K BTU propane heater I used in a really rusted out cab of a grove crane I used to run. I would advise against using that type of unit in a full enclosed cab without proper ventilation for the exhaust. Carbon monoxcide poisioning could be possible. If this payloader has a good charging /battery system there are electrical heaters that could do the job out there. I do not mean the cheap disposible models either.